This application claims the priority of German patent document 199 24 941.5, filed May 31, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and a control system for setting a presettable vehicle desired speed, in which a cruise control system generates signals for controlling the engine or the transmission, based on vehicle state and operating variables.
Motor vehicle cruise control systems are known which automatically maintain a desired speed that is preset by the driver and is calculated via an automatic determination system. Such cruise control systems include both Tempomat functions, for which the vehicle is intended to maintain a minimum speed selected by the driver, and limiter functions, for which the vehicle must not exceed a maximum speed which is preset by the driver or is determined automatically in proximity control systems. In order to implement the limiter function, control signals are produced in the cruise control system, based on the maximum speed and the actual vehicle state variables, in particular the actual vehicle speed. By means of these signals the engine, the transmission or the wheel brakes are adjusted in order to maintain the desired speed.
In German patent document DE 44 07 082 A1 a transmission control signal interrupts signal interrupts the regular transmission shift logic in order to brake the vehicle from the present speed to a desired maximum speed, determined, for example, based on the distance from a vehicle travelling in front. For this purpose, a gear change to a lower gear is implemented, thus achieving a lower gear ratio with a greater level of deceleration. The gear limit is cancelled again once the desired maximum speed is reached, and the regular transmission shift logic is restored.
When the actual vehicle speed approaches the desired speed, the desired speed may be briefly overshot or undershot during the transient process. This effect, which is known as "hunting", occurs with all standard, linear standard regulators with a proportional, integral and/or differential element. The amplitude of the overshoot in this case increases with the speed gradient in the region where the desired speed is exceeded; that is, the more quickly the actual vehicle speed approaches the desired speed.
Drivers often find overshooting to be unpleasant, and irritating. Furthermore, there is a risk that gear changes will be caused in the automatic transmission in order to compensate for the overshoot, with the gear immediately changing back again when the reverse oscillation occurs. Such immediately successive gear changes are likewise regarded as disturbing.
German patent document DE 37 01 331 A1 discloses a method and a control system for setting a vehicle desired speed, in which a speed substitute value that differs from the desired speed is preset in order to avoid unnecessarily high accelerations during regulation of the vehicle speed. The substitute speed is calculated in accordance with a predetermined function, which includes the actual vehicle speed. The substitute speed is continuously updated in each iteration of the method, so that it also ever more closely matches the desired speed as the method progresses, based on the fact that the actual vehicle speed approaches the desired speed. Thus, the substitute speed is continuously matched to the desired speed, and the offset between the desired speed and the substitute speed is continuously reduced. During this process of continuous matching of the vehicle speed to the desired speed, a new parameter substitute for ignition, injection, air supply and the transmission must be determined and provided for each newly determined value of the substitute speed, which results in a permanent change to the corresponding control elements. Owing to the different reaction times, the matching of these control elements must be synchronized to the reaction time of the slowest reacting control element. (Otherwise, it might be impossible to set a stoichiometric fuel/air mixture.) This synchronization results in a delay to the setting of the desired speed. A further disadvantage of this system is the high calculation and control complexity.
German patent document DE 42 42 169 A1 deals with the problem of reactivating a cruise control system once it has been switched off. After reactivation, the nominal speed is changed in progressive steps from the previous nominal value to the actual nominal value. As the step-by-step change is made from the original nominal speed to the currently applicable nominal speed, the same problems occur as those with the control system according to DE 37 01 331 A1. That is, the permanent matching of ignition, injection, air supply and transmission while at the same time ensuring an optimum fuel/air mixture.
In a cruise control system according to German patent document DE 196 04 220 A1, a substitute speed is first preset in order to match the vehicle speed to the desired speed. This substitute speed is a comparatively complex, non-linear function, which arises from the fact that the speed change in this document is intended to follow a predetermined optimization criterion. The non-linear function for the substitute speed likewise means permanent matching of engine variables and transmission variables, however; furthermore, this function does not prevent overshoot beyond the final desired speed, since the substitute speed goes beyond the desired speed, and the actual vehicle speed follows the substitute speed.
A control system which is comparable to that of German patent document DE 196 04 220 A1 and having a highly non-linear profile for the nominal speed is also disclosed in German patent document DE 35 10 174 C2.
One object of the invention is to improve driving safety and convenience in vehicles with a speed control unit.
Another object of the invention is to avoid overshoots when approaching the desired speed.
These and other objects and advantages are achieved by the method and system according to the invention, in which control is based on a substitute speed rather than the desired speed, with the substitute speed being formed by applying an offset to the desired speed. The offset is selected such that the substitute speed is above the desired speed if the actual vehicle speed at the time when the cruise control is activated is also above the desired speed. In this manner, the substitute speed is reached first, when approaching the desired speed. In a corresponding manner, if the actual vehicle speed is below the desired speed, the substitute speed is also set to a value which is below the desired speed.
This process offers the advantage that the transient variations takes place about the value of the substitute speed. The offset between the desired speed and the substitute speed may be selected in such a way that overshooting amplitudes fall within the interval between the substitute speed and the desired speed; thus, the desired speed is not exceeded. In this manner, the transient process, which normally takes place about the value of the desired speed, is shifted to the substitute speed.
The invention may be used not only starting from higher vehicle speeds but also starting from lower vehicle speeds than the desired speed, so that the desired speed can be approached both from above and below. The invention may be used both for Tempomat functions and for limiter functions.
The offset which is used to define the substitute speed is expediently taken from a family of characteristics, as a function of the actual speed changes of the vehicle and, advantageously also as a function of the actual transmission gear. The actual vehicle speed at the time when the substitute speed is applied may also be taken into account, as a further limit. If appropriate, the offset is limited to a maximum value.
Various conditions may be taken into account for applying and discontinuing the substitute speed on which the cruise control is based. For application, the speed change of the vehicle--acceleration or deceleration--and, advantageously, the vehicle speed as well, are expediently compared with reference values. In particular, a check is carried out to determine whether the actual vehicle speed is initially outside a speed threshold value, in order to allow activation of the control function taking account of the substitute value only in the situation when the vehicle speed is sufficiently far away from the desired speed. A check is then carried out to determine whether the vehicle speed is within the interval between the threshold value and the desired speed, and whether the speed change exceeds a corresponding threshold value. Application preferably takes place when all the conditions are satisfied.
On discontinuation, the speed offset is set to zero, so that the substitute speed and the desired speed coincide. As a condition for disconnection, it is possible to take account of whether the regulator output of the control system is changing sign, as is the case when the regulator output value passes through zero. Furthermore, it is also desirable to check whether the vehicle acceleration reverses to a deceleration or whether the deceleration reverses to an acceleration, which likewise occurs when the speed change passes through zero.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.